you can't write the book for everyone else - you have to write it for you. My $0.02
I guess a book written for yourself and not others would be worth $0.02
The way I see it is the market is not really huge, but except for Jerry Winters book I do not think there are any other books.
Would get the C# crowd interested.
There is a ton of great resources for .NET out so it might not be worth it to cover much .NET?
Although you see comments all the time as if .NET does not exists except for AutoCAD development.
I would rather read 10 pages explaining how it works than a bunch of examples.
What I think is really missing is well written, well ordered, book explaining the API, which I guess takes a good understanding of ObjectARX.
So I guess you will send the first draft within in month?